The process of membrane uptake of lipids by intestinal epithelial cells is being studied in vitro with rats. Short term exposure of everted intestine to isotopes of fatty acids, monoglycerides, cholesterol, or molecular analogs provides data from which permeation coefficients are calculated. Other data required for these calculations are chemical activities of dissolved lipid in micellar solutions which are identical to those used for uptake determinations. These data are determined for each solute using a partitioning system. Solutes are chosen to provide information concerning the molecular nature of the intestinal brush border membrane and the role of various determinants of membrane permeation.